Universe Today

The stars life sequence, ending with the formation of a black hole. Image credit: Nicolle Rager Fuller/NSF Click to enlarge
Just a few hundred millions years after the Big Bang, a massive star exhausted its fuel, collapsed as a black hole, and exploded as a gamma ray burst. The radiation from this catastrophic event has only now reached Earth, and astronomers are using it to peer back to the earliest moments of the Universe. The burst, named GRB 050904, was observed by NASA’s Swift satellite on September 4, 2005. One unusual thing about this burst is that it lasted for 500 seconds – most are over in a fraction of that time.

It came from the edge of the visible universe, the most distant explosion ever detected.

In this week’s issue of Nature, scientists at Penn State University and their U.S. and European colleagues discuss how this explosion, detected on 4 September 2005, was the result of a massive star collapsing into a black hole.

The explosion, called a gamma-ray burst, comes from an era soon after stars and galaxies first formed, about 500 million to 1 billion years after the Big Bang. The universe is now 13.7 billion years old, so the September burst serves as a probe to study the conditions of the early universe.

“This was a massive star that lived fast and died young,” said David Burrows, senior scientist and professor of astronomy and astrophysics at Penn State, a co-author on one of the three reports about this explosion published this week in Nature. “This star was probably quite different from the kind we see today, the type that only could have existed in the early universe.”

The burst, named GRB 050904 after the date it was spotted, was detected by NASA’s Swift satellite, which is operated by Penn State. Swift provided the burst coordinates so that other satellites and ground-based telescopes could observe the burst. Bursts typically last only 10 seconds, but the afterglow will linger for a few days.

GRB 050904 originated 13 billion light years from Earth, which means it occurred 13 billion years ago, for it took that long for the light to reach us. Scientists have detected only a few objects more than 12 billion light years away, so the burst is extremely important in understanding the universe beyond the reach of the largest telescopes.

“Because the burst was brighter than a billion suns, many telescopes could study it even from such a huge distance,” said Burrows, whose analysis focuses mainly on Swift data from its three telescopes, covering a range of gamma-rays, X-rays, and ultraviolet/optical wavelengths, respectively. Burrows is the lead scientist for Swift’s X-ray telescope.

The Swift team found several unique features in GRB 050904. The burst was long–lasting about 500 seconds–and the tail end of the burst exhibited multiple flares. These characteristics imply that the newly created black hole didn’t form instantly, as some scientists have thought, but rather it was a longer, chaotic event.

Closer gamma-ray bursts do not have as much flaring, implying that the earliest black holes may have formed differently from ones in the modern era, Burrows said. The difference could be because the first stars were more massive than modern stars. Or, it could be the result of the environment of the early universe when the first stars began to convert hydrogen and helium (created in the Big Bang) into heavier elements.

GRB 050904, in fact, shows hints of newly minted heavier elements, according to data from ground-based telescopes. This discovery is the subject of a second Nature article by a Japanese group led by Nobuyuki Kawai at the Tokyo Institute of Technology.

GRB 050904 also exhibited time dilation, a result of the vast expansion of the universe during the 13 billion years that it took the light to reach us on Earth. This dilation results in the light appearing much redder than when it was emitted in the burst, and it also alters our perception of time as compared to the burst’s internal clock.

These factors worked in the scientists’ favor. The Penn State team turned Swift’s instruments onto the burst about 2 minutes after the event began. The burst, however, was evolving as if it were in slow motion and was only about 23 seconds into the bursting. So scientists could see the burst at a very early stage.

Only one other object–a quasar–has been discovered at a greater distance. Yet, whereas quasars are supermassive black holes containing the mass of billions of stars, this burst comes from a single star. The detection of GRB 050904 confirms that massive stars mingled with the oldest quasars. It also confirms that even more explosions of distant stars–perhaps from the first stars, theorists say–can be studied through a combination of observations with Swift and other world-class telescopes.

“We designed Swift to look for faint bursts coming from the edge of the universe,” said Neil Gehrels of NASA Goddard Space Flight Center in Greenbelt, Maryland, Swift’s principal investigator. “Now we’ve got one and it’s fascinating. For the first, time we can learn about individual stars from near the beginning of time. There are surely many more out there.”

Swift was launched in November 2004 and was fully operational by January 2005. Swift carries three main instruments: the Burst Alert Telescope, the X-ray Telescope, and the Ultraviolet/Optical Telescope. Swift’s gamma-ray detector, the Burst Alert Telescope, provides the rapid initial location, was built primarily by the NASA Goddard Space Flight Center in Greenbelt and Los Alamos National Laboratory, and was constructed at GSFC. Swift’s X-Ray Telescope and UV/Optical Telescope were developed and built by international teams led by Penn State and drew heavily on each institution’s experience with previous space missions. The X-ray Telescope resulted from Penn State’s collaboration with the University of Leicester in England and the Brera Astronomical Observatory in Italy. The Ultraviolet/Optical Telescope resulted from Penn State’s collaboration with the Mullard Space Science Laboratory of the University College-London. These three telescopes give Swift the ability to do almost immediate follow-up observations of most gamma-ray bursts because Swift can rotate so quickly to point toward the source of the gamma-ray signal.

Original Source: PSU News Release

Artist’s concept symbollically represents early universe organic complex compounds. Image credit: NASA/JPL Click to enlarge
By studying distant galaxies with the Spitzer space telescope, researchers have come to the conclusion that the intense radiation of infant galaxies was very destructive to life’s building blocks. Shortly after the Big Bang, these young galaxies blazed in star formation, but they had very few organic molecules – which are quite common in older galaxies. Even through these organic molecules will be forming in young stars, their intense radiation destroys them again.

The components of life may have been under attack in the hostile environments of the universe’s first galaxies, say astronomers using NASA’s Spitzer Space Telescope.

A science team led by graduate student Yanling Wu of Cornell University, Ithaca, N.Y., recently came to this conclusion after studying the formation and destruction of polycyclic aromatic hydrocarbons molecules (PAHs) in more than 50 blue compact dwarf (BCD) galaxies. These organic molecules, comprised of hydrogen and carbon, are believed by many scientists to be among the building blocks for life.

“One of the outstanding problems in astronomy today is whether complex organic molecules of hydrogen and carbon, similar to those responsible for life on Earth, are present in the early universe,” says Wu.

According to Wu, mature massive galaxies like our Milky Way formed from the merging of smaller galaxies, probably about the size of nearby BCD galaxies. Since current technology is not sensitive enough to easily identify and study in detail the universe’s first galaxies, astronomers must infer the physical properties of the early structures by observing similar nearby galaxies like BCDs.

“We believe that BCD galaxies are similar to the universe’s first galaxies because they are infant galaxies, actively forming stars, and are not very chemically polluted,” said Wu.

Because most atomic elements other than hydrogen and helium are born from the death of stars, astronomers suspect that in the first few million years after the big bang galaxies were not “chemically polluted” with elements other than hydrogen and helium. In astronomy, these relatively unpolluted galaxies are said to have low metallicity.

The BCD galaxies’ blue colors tell astronomers that these structures are actively forming massive stars. By logically combining the galaxy’s blue color with the fact that it is low in metals, astronomers can infer that this is a young galaxy.

In her research, Wu found that nearby BCD galaxies with lowest metallicity also had little or no PAHs. As the galaxies became more chemically polluted, more traces of PAHs were found. She notes that this phenomenon makes sense because heavy metal elements like carbon are formed from the death of stars, and some of these galaxies may just be too “young” to have produced enough carbon to create PAHs.

However, in some of the BCD galaxies where the conditions allow for the formation of PAHs, Wu found that those molecules were being destroyed by intense ultraviolet radiation from the young massive stars.

“Because BCD galaxies are metal poor and very compact, the intense ultraviolet radiation from young stars will destroy PAH molecules even if they are formed,” says Wu. “The threshold for when these PAH molecules stop being destroyed is still uncertain.”

“This leads to an interesting paradox, where the young stars responsible for the formation of PAHs may also be the main culprit of their destruction,” adds co-author Dr. Vassilis Charmandaris, of the University of Greece, Heraklion.

The organic PAHs were detected using Spitzer’s Infrared Spectrometer (IRS).

“Yanling has made significant progress in a research area first opened by International Space Observatory ,” says Dr. Jim Houck of Cornell University. Houck is Wu’s academic advisor and a co-author of the paper. He is also the Principal Investigator for Spitzer’s IRS instrument and played a vital role in its creation.

“With Spitzer, Yanling is able to extend BCDs observations to a much larger sample; the new results provide a glimpse into the formation of galaxies in the early Universe,” he adds.

Wu’s paper will be published in a March issue of Astrophysical Journal. For more information on this discovery please listen to the podcast interview with Yanling Wu.

Original Source: Spitzer Space Telescope

The current understanding of a pulsar. Image credit: Jodrell Bank Observatory. Click to enlarge
Astronomers have discovered a very unusual pulsar that seems to switch off from time to time. It looks like a normal pulsar for about a week, blasting out radio waves, and then goes silent for about a month. This pulsar is slowing down its rate of rotation, but this deceleration increases when it’s active. This braking mechanism is related to the powerful radio emissions. During its active phase, a wind of particles is spewed off, stealing some of its rotational energy.

Astronomers using the 76-m Lovell radio telescope at the University of Manchester’s Jodrell Bank Observatory have discovered a very strange pulsar that helps explain how pulsars act as ‘cosmic clocks’ and confirms theories put forward 37 years ago to explain the way in which pulsars emit their regular beams of radio waves – considered to be one of the hardest problems in astrophysics. Their research, now published in Science Express, reveals a pulsar that is only ‘on’ for part of the time. The strange pulsar is spinning about its own axis and slows down 50% faster when it is ‘on’ compared to when it is ‘off’.

Pulsars are dense, highly magnetized neutron stars that are born in a violent explosion marking the death of massive stars. They act like cosmic lighthouses as they project a rotating beam of radio waves across the galaxy. Dr Michael Kramer explains, “Pulsars are a physicist’s dream come true. They are made of the most extreme matter that we know of in the Universe, and their highly stable rotation makes them super-precise cosmic clocks – but, embarrassingly, we do not know how these clocks work. This discovery goes a long way towards solving this problem.”

The current understanding of a pulsar. The central neutron star is highly magnetised and emits a radio beam along its magnetic axis, which is inclined to the rotation axis. The strong magnetic field eventually leads to the extraction of particles from the surface, filling the surrounding, so-called magnetosphere with plasma. The size of the magnetosphere is given by the distance where plasma co-rotation reaches the speed of light, the so-called light-cylinder. The plasma creating the radio emission eventually leaves the light cylinder as a pulsar wind, which provides a torque onto the pulsar, contributing about 50% to its observed slow-down in rotation.

The research team, led by Dr Kramer, found a pulsar that is only periodically active. It appears as a normal pulsar for about a week and then “switches off” for about one month before emitting pulses again. The pulsar, called PSR B1931+24, is unique in this behaviour and affords astronomers an opportunity to compare its quiet and active phases. As it is quiet the majority of the time, it is difficult to detect, suggesting that there may be many other similar objects that have, so far, escaped detection.

Prof Andrew Lyne points out that, “After the discovery of pulsars, theoreticians proposed that strong electric fields rip particles out of the neutron star surface into a surrounding magnetised cloud of plasma called the magnetosphere – but, for nearly 40 years, there had been no way to test whether our basic understanding was correct.”

The University of Manchester astronomers were delighted when they found that this pulsar slows down more rapidly when the pulsar is on than when it is off. Dr Christine Jordan points out the importance of this discovery, “We can clearly see that something hits the brakes when the pulsar is on.”

This breaking mechanism must be related to the radio emission and the processes creating it and the additional slow-down can be explained by a wind of particles leaving the pulsar’s magnetosphere and carrying away rotational energy. “Such a braking effect of the pulsar wind was expected but now, finally, we have observational evidence for it” adds Dr Duncan Lorimer.

The amount of braking can be related to the number of charges leaving the pulsar magnetosphere. Dr Kramer explains their surprise when it was found that the resulting number was within 2% of the theoretical predictions. “We were really shocked when we saw these numbers on our screens. Given the pulsar’s complexity, we never really expected the magnetospheric theory to work so well.”

Prof Lyne summarized the result: “It is amazing that, after almost 40 years, we have not only found a new, unusual, pulsar phenomenon but also a very unexpected way to confirm some fundamental theories about the nature of pulsars.”

Original Source: PPARC News Release